Full information from an X-ray image can include absolute values of the lengths of extremities of a patient being known. Such absolute data can be indispensable, particularly in the planning of an operation.
A lead ruler has been used until now to provide a scale of length. The lead ruler is arranged next to the patient or else fastened to him/her and is also reproduced on the X-ray image. In particular, fastening the lead rule to the patient is laborious and costly and requires the cooperation of the patient. The use of a lead ruler also has the disadvantage that it is difficult to compile multiple images and then determine lengths covering all the images. If therefore lengths are to be derived from the images, recordings of the whole extremity are made at once over a large area, e.g. whole-leg recordings using a film cassette which is 90 or 120 cm long.
From DE 31 28 380 A1, an X-ray diagnostic installation for X-ray tomographic images is known. Certain layers of a patient lying on a patient table are especially highlighted by an X-ray image. In order to calculate the enlargement, a layer height computer is provided which uses the geometric data for the tomograph as an input variable. A marking for the dimensions in the layer plane can be faded into the image. The geometric data for the tomograph is displayed as given and does not have to be measured specifically.
From US 2005/0007387 A1 a device, method and program for the graphical representation of X-ray images are known, wherein in an embodiment for defining an image enlargement the intercept theorems are used. It is not disclosed how the input variables for the intercept theorems are determined.
In DE 101 60 532 C1, a method and a device are known for the three-dimensional locating of an object in a body. The focus of interest is on determining the vertical position of the object. Instead of using a ruler, the object is displaced around a displacement path and the projection of the displacement path determined in an X-ray image. From the intercept theorems, the vertical position of the object can be determined with the aid of the displacement path and the projection and the distance of the radiation source for the X-ray image from the image plane of the projection image.
From EP 1 349 098 A1 and U.S. Pat. No. 6,792,071 B2 methods are known for performing geometric measurements on digital radiological images, in which so-called graphical templates are used. These templates also incorporate as geometric parameters the distances between X-ray source, patient and detector.